Automatic gain control based on multiple input references in a video decoder

ABSTRACT

A video decoder ( 10 ) for receiving an input video signal and producing output signals for a digital display ( 12 ), and involving automatic gain control (AGC) functions ( 20, 28 ), is disclosed. For each of the front-end and back-end AGC functions ( 20, 28 ), the AGC gain value applied is modified according to measurements of several attributes of the video signal, such as sync height, color burst amplitude, composite signal peak amplitude, and luma signal peak amplitude. The measurements are made, and the AGC gains are modified, on a frame-by-frame basis. Preferably, the AGC is modified using the one of the signal attributes that indicates the lowest gain. The back-end AGC function ( 28 ) modifies its gain to unity if the front-end AGC function ( 20 ) selected the luma signal peak amplitude for modifying its gain. A method for adjusting the rate of change of the AGC gain is also disclosed, in which gain increases are delayed and slowed following an AGC gain decrease because of an image-dependent signal attribute.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

This invention is in the field of digital video systems, and is morespecifically directed to video decoders.

Modern high-performance video and television displays, especially plasmaand liquid-crystal (LCD) displays, are adapted to receiving digitalsignals corresponding to the information to be displayed. These digitalinputs indicate the intensity, typically by component, to be displayedby each picture element (“pixel”) of the display. For example, moderncomponent video signals include a component value for each of the pixelattributes of luma (“Y”), chroma-blue (“Pb”), and chroma-red (“Pr”). Asa result, modern high resolution displays, having over one thousandpixels in each dimension, with each pixel responsive to as much as atwenty-four bit digital signal, are able to render high fidelity imagesat real time data rates.

As known in the art, video inputs are communicated and stored in a widevariety of formats. Broadcast television signals are still communicatedin the analog domain, and these analog signals are communicatedaccording to multiple standards around the world. In addition, videosignals from other sources are now also available for display on digitaldisplays. These other sources include cable and satellite digital videotransmissions, video cameras, and video playback devices such as DVDplayers and video cassette recorders. In any case, these signals areoften in the form of “composite” video signals, in that the colorsignals are communicated in the form of “luma” and “chroma” (or colordifference) signals, rather than as intensity levels for each of theprimary component colors. These signals are also typically analogsignals. Examples of the applicable standards for conventional videosignals include the well-known NTSC (National Television SystemsCommittee), PAL, and SECAM composite video signal standards.

Video decoder functions are now commonly used in many high-performancedigital display and television systems for receiving video signals fromthese various sources and converting the video signals into a digitalform for display. For example, a so-called “set-top box” for receivingcable or satellite digital video transmissions and for driving a digitalvideo display typically includes a video decoder function. Modernset-top boxes also often have auxiliary inputs for receiving videosignals from other sources, from which the video decoder in the set-topbox generates the digital video output signals. Other systems thatinclude a video decoder function include video decoder cards forpersonal computers, personal video recorders (PVRs) for digitallyrecording broadcast, cable, or satellite transmissions for laterviewing, digital video projectors, digital VCRs and DVD recorders, videoor home theater receivers, and indeed digital television sets andcomputer displays that are themselves (i.e., without an external set-topbox) capable of digitally displaying video output from conventionalanalog input signals.

Recently, the video decoder function in these systems has beenimplemented within a single integrated circuit. Examples of conventionalvideo decoder integrated circuits include the TVP5145 digital videodecoder available from Texas Instruments Incorporated, and the SAA7115video decoder available from Philips Semiconductors.

In the decoding of input signals into digitally displayable outputsignals, conventional video decoders typically apply gain to the decodedsignal, so that the digital output signals have amplitudes that fit wellwithin the dynamic range of the display device. Conventional videodecoders apply this gain by way of automatic gain control (AGC), whichin its general sense amplifies a varying input voltage using a gain thatdepends on the input voltage itself. AGC circuits and functions thusautomatically control the amplifier gain so that the output voltageremains constant or within a predetermined dynamic output range.

In conventional video decoders for converting analog video signals intodigital display signals, the AGC function measures the amplitude of theanalog signal at a known time within the periodic signal, and adjuststhe gain of its output signal based on that amplitude. Typically,conventional video decoders sample the analog video signal at itssynchronization (“sync”) level, which is a portion of the analog signalin the horizontal blanking interval that is not displayable by thedisplay but which is used to synchronize the displayable portions of thesignal with the display scan lines. The sync “height”, or amplitude(i.e., the difference between the sync level and a reference level, suchas the “back porch” level), is used in these conventional video decodersas the control input to an AGC function, in response to which the AGCgain of the video decoder is set.

It has been observed, in connection with this invention, that the syncamplitude may not be representative of the actual amplitude of the videoinformation to be displayed. For example, in the well-known NTSCstandard, the nominal sync amplitude is −40 IRE, but this sync pulse isfrequently compressed or clipped. If this clipping occurs, the clippedor compressed sync pulse received by the digital video decoder will beat a lower amplitude than its ideal amplitude. The AGC function in theconventional video decoder will, as a result, set its gain undesirablyhigh in an attempt to compensate for the lower amplitude sync height,but this gain will be too high for the video signal itself (which wasnot clipped). The resulting images displayed will tend to be saturated,or too bright.

By way of further background, the SAA7115 video decoder available fromPhilips Semiconductors includes different AGC gain adjustment ratesdepending upon whether the gain is to be increased or decreased.Specifically, the SAA7115 video decoder unconditionally decreases itsAGC gain rapidly and increases its AGC gain slowly. It is believed thatthis differential AGC gain adjustment rate approach is intended toensure that the gain applied to bright image scenes following darkscenes of brief duration do not saturate.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a video decoderthat automatically controls its gain to provide accurate fidelity in thedisplayed images.

It is a further object of this invention to provide such a video decoderin which automatic gain control (AGC) is not deceived by the clipping ofsync level or other portions of the input signal.

It is a further object of this invention to provide such a video decoderand AGC function in which correction of over-amplified portions of thesignal is also performed to further improve the fidelity of the outputdigital video signals.

It is a further object of this invention to provide such a video decoderand AGC function in which the rate of change in the gain is controlledso that convergence to the true image sequence is optimized.

Other objects and advantages of this invention will be apparent to thoseof ordinary skill in the art having reference to the followingspecification together with its drawings.

This invention may be implemented into a video decoder integratedcircuit in which automatic gain control (AGC) is performed on anincoming video signal based upon multiple attributes of that signal.According to a preferred embodiment of the invention, AGC is appliedboth prior to and after the separation of the input video signal intoits components, where each AGC gain level is controlled using a selectedone of multiple attributes of the signal.

According to another aspect of this invention, increases in the AGC gainare controlled in response to the signal attribute that caused the mostrecent decrease in AGC gain. More specifically, if the AGC gain isreduced due to an image-dependent attribute of the signal, subsequentincreases in the AGC gain are delayed, and then more slowly increased,relative to the AGC gain increases that are permitted if the most recentAGC gain reduction were due to image-independent attributes of thesignal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is an electrical diagram, in block form, of a display systemincluding a video decoder constructed according to the preferredembodiment of the invention.

FIG. 2 is an electrical diagram, in block form, of a video decoderconstructed according to the preferred embodiment of the invention.

FIG. 3 is a data flow diagram illustrating the operation of thefront-end AGC function in the video decoder constructed according to thepreferred embodiment of the invention.

FIG. 4 is a flow chart illustrating the operation of the updating of thefront-end AGC gain value, according to the preferred embodiment of theinvention.

FIG. 5 is a timing diagram illustrating an exemplary input videowaveform used by the video decoder according to the preferred embodimentof the invention.

FIG. 6 is a data flow diagram illustrating the operation of the back-endAGC function in the video decoder constructed according to the preferredembodiment of the invention.

FIG. 7 is a flow chart illustrating the operation of the updating of theback-end AGC gain value, according to the preferred embodiment of theinvention.

FIG. 8 is a flow chart illustrating the operation of the control offilter coefficient values for the increasing of AGC gain values,according to the preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in connection with its preferredembodiment, namely as implemented into a video decoder integratedcircuit, and a system utilizing that integrated circuit. However, it iscontemplated that this invention may have benefit in other applicationsother than the specific implementation described in this specification.Accordingly, it is to be understood that the following description isprovided by way of example only, and is not intended to limit the truescope of this invention as claimed.

FIG. 1 illustrates a video display system into which the preferredembodiment of the invention can be implemented. In this example,multiple video sources 8 a through 8 n provide video signals, accordingto one or multiple different standards or formats, to video decoder 10.In turn, video decoder 10 decodes the incoming video signals andpresents a digital video signal to video display 12, in a format.Examples of video sources 8 a through 8 n include conventional videosources such as cable and satellite digital video services, videocameras, video playback devices such as DVD players and video cassetterecorders, personal computers, and the like. Examples of the standardsor formats of the video signals include composite signals such as thoseaccording to the NTSC, PAL, and SECAM standards, high definition TVsignals, and digital video signals. Of course, the system may includeany number of video sources 8, ranging from a single source to several.

Video decoder 10 as shown in the system of FIG. 1 may be implemented invarious parts of the system. For example, video decoder 10 may beimplemented within a set-top box, capable of receiving inputs frommultiple video sources and delivering digital video signals to display12. Alternatively, video decoder 10 may be physically implemented withinone or more of video sources 8 that are capable of providing digitalvideo signals directly to display 12; an example of this implementationis on the graphics card of a personal computer or workstation. Anotherexample is the incorporation of video decoder 10 within a DVD recorderor playback system, or digital VCR. It is contemplated that these andother arrangements are well-known to those in the art having referenceto this specification. For purposes of this invention, the physicallocation of video decoder 10 is not particularly important.

FIG. 2 illustrates, in block form, the construction of video decoder 10according to the preferred embodiment of the invention. Input interface14 receives one or more input video signals from video sources 8 athrough 8 n (FIG. 1), and buffers and level-converts those input signalsin the conventional manner, according to the nature of the signalsreceived and the requirements of downstream functions within videodecoder 10. The input video signals are then applied to analog front-end15, which performs functions including coarse gain control,analog-to-digital conversion, and also digital automatic gain controlaccording to this preferred embodiment of the invention as will bedescribed below.

More specifically, analog front-end 15 includes coarse gain controlfunctions 16 a, 16 b, which apply a coarse AGC correction to the signalsreceived by interface 14. Offset compensation (not shown) may also beapplied at this point. As will be described in further detail below,coarse gain control functions 16 operate in the analog domain, yet aredigitally controlled. The analog output of coarse gain control functions16 a, 16 b are then applied to analog-to-digital converters (ADCs) 18 a,18 b, respectively, which sample the amplified analog signals andconvert them into the digital domain, as known in the art. Preferably,ADCs 18 are at least of ten-bit resolution, for high-fidelity andhigh-definition video display functionality. The video signals, now inthe digital domain in the form of sampled datastreams, are then appliedto respective digital front-end AGC functions 20 a, 20 b, which apply afiner AGC correction to the digital video signals. According to thepreferred embodiment of the invention, digital front-end AGC functions20 a, 20 b set their gains by way of feedback measurements of a selectedsignal attribute, as will be described in further detail below. Offsetcompensation (not shown) may also be performed after digital front-endAGC functions 20. Input format circuit 24 comprehends the blankingintervals and other synchronization information contained within thevideo signals, for the particular signal standard or format receivedfrom digital front-end AGC functions 20 a, 20 b, and strips thedisplayable information (e.g., luma and chroma) from the incoming videosignals, in the conventional manner.

Separation and filtering function 26 performs the bulk of the videodecoding operation, more specifically the luma and chroma separationfunction. In the case of conventional composite video signals, forexample according to the NTSC (National Television System Committee)standard, the luma signal (Y) is a wideband signal that isrepresentative of the overall brightness at each corresponding locationof the image to be displayed, and as such is the only signal requiredfor display of a monochrome image. The chroma signal is, according tothis standard, communicated by two quadrature-phase signal componentsthat are modulated onto a subcarrier at about 3.6 MHz, each componentcorresponding to a “color difference”. In the NTSC standard, thein-phase component is B−Y (blue minus luma) and the quadrature-phasecomponent is R−Y (red minus luma). Separation and filtering function 26isolates and separates the luma and chroma signal components from thecomposite signal. Separation and filtering function 26 also applies thedesired filtering to the separated signals, for example by way of anadaptive three-line or four-line comb filter or the like, to reducecross-luma and cross-chroma artifacts, as known in the art.

According to the preferred embodiment of the invention, back-end AGCfunction 28 applies another digital automatic gain control (AGC) to theseparated component signals produced by separation and filteringfunction 26. The manner in which back-end AGC function 28 performs thisfunction will be described in further detail below. The modified signalfrom back-end AGC function 28 is forwarded to output formatter 30, whichdemodulates the chroma components and, by way of conventional linearcombination or the like, separates the chroma signals into theappropriate color component signals (e.g., Pb, Pr). Output formatter 30also arranges the resulting digital component values into theappropriate components of the desired format for output to display 12.Examples of typical digital video output formats applied by outputformatter 30 include the 4:2:2 format at various data rates, and alsothe format specified by the ITU-R.BT.656 standard, with embedded syncs,at various bit rates.

Video decoder 10 includes support circuitry and functions forcontrolling its operation, and for providing access to external devices,for example to program the various options under which video decoder 10is to process the video signals. Clock circuitry 32 receives orgenerates a system clock signal, and typically includes phase-lockedloops for generating internal clock signals for operation of the variousfunctions within video decoder 10. Digital processor 35 is preferably aprogrammable logic device, for example a microprocessor or digitalsignal processor (DSP), for performing the necessary digital logic andcontrol operations involved in video decoder 10, including the controlof front-end AGC function 20 and back-end AGC function 28. Digitalprocessor 35 is bidirectionally coupled to host interface 36, by way ofwhich control signals are received, and monitoring and status signalsare output, by video decoder 10. For example, it is contemplated thatconfiguration commands can be applied to video decoder 10 via hostinterface 36 to set the operational environment of video decoder 10,including various parameters related to the updating of AGC gain valuesfor front-end AGC function 20 and back-end AGC function 28, as will bedescribed in further detail below.

Other processing functions (not shown in FIG. 2) may also be includedwithin video decoder 10. For example, as known in the art, text andother information is often encoded within the vertical blanking interval(VBI) of a television signal. Examples of this information includesclosed captioning or other subtitles associated with the content beingdisplayed. This information may be decoded by a VBI processing circuitwithin video decoder 10, operating in combination with input formatcircuit 24. Circuitry for processing copy protection information, forexample according to the MACROVISION copy protection scheme, may also beincluded within video decoder 10, also operating in combination withinput format circuit 24. These and other alternative functions andimplementations are contemplated to be within the capability of those ofordinary skill in the art having reference to this specification.

Referring now to FIG. 3, the operation of front-end AGC function 20 fora single one of the channels processed by video decoder 10, according tothe preferred embodiment of the invention, will now be described indetail. As shown in FIG. 3, the input video signal (e.g., from inputinterface 14) is amplified by coarse AGC stage 16 in front-end AGCfunction 20. The gain (gain₁₆) applied by coarse AGC stage 16 follows aconventional linear AGC characteristic:gain ₁₆ =a ₃ +b ₃ N _(CG-1)  (1)where N_(CG-1) is a digital control value (e.g., a four-bit value) forsetting the coarse gain, and where a₃ is the y-intercept and b₃ is theslope of the linear AGC characteristic. Again, as mentioned above,offset compensation (not shown) may also be performed in the analogdomain. ADC 18 converts the output of coarse AGC gain stage 16 to thedigital domain.

Digital front-end AGC stage 20 similarly applies AGC to the digitalsignal. According to this embodiment of the invention, the gain valuegain₂₀ of digital front-end AGC stage 20 similarly follows a linear AGCcontrol characteristic:gain ₂₀ =a ₁ +b ₁ G _(n-1)  (2)where G_(n-1) is a digital control value (e.g., of eight bits or more,for precision) for setting the AGC gain, and where a₁ and b₁ are they-intercept and slope, respectively, of the linear AGC control function.In one exemplary implementation of this invention, the product of thegain applied by front-end AGC stage 20 with the previously-appliedcoarse AGC gain may vary in a range from 0.25 to 4.00, each stageapplying a gain ranging from 0.50 to 2.00. The amplified input videosignal is then clipped, in the conventional manner, by clip stage 42,and is then forwarded along the data path of video decoder 10. As shownin FIG. 2, the amplified and clipped input signal is converted into thedigital domain by one or both of ADCs 22, and processed according to itsinput format by input format function 24. Ultimately, the input signal(now in the digital domain) is then processed by separation andfiltering function 26, by way of which the luma and chroma signals areseparated, and eventually demodulated and converted into the digitalcomponent values as described above.

According to the preferred embodiment of the invention, front-end AGCstage 20 adjusts its current gain control value G_(n-1) to provide anupdated gain control value G_(n) based on the measurements of multipleattributes in the video signal. As shown in FIG. 3, these measurementsinclude measurements of the input signal after gain stage 40 and clipfunction 42, prior to luma and chroma separation. Some of theseattributes may be considered as indirectly correlating to the outputsignal, while others directly correlate to the output. Thesemeasurements of the various attributes of the signal are applied to gainupdate calculator 46, by way of which the next gain control value G_(n)is derived. In addition, certain parameters regarding the front-end AGCgain are communicated to back-end AGC function 28, as shown in FIG. 3.The particular circuit function within analog front-end 15 that makesthese measurements and update calculations may be implemented by customlogic, or by programmable logic under the control of a software routine,or alternatively these measurements and calculations may be made byother circuitry in video decoder 10, for example digital processor 35(FIG. 2) and forwarded to analog front-end 15 as a control signal. Whilethe following description will refer to functions within analogfront-end 15 as performing gain update and storage functions, thoseskilled in the art having reference to this specification will realizethat such functions may be implemented in any one of a number of ways,without departing from this invention.

The manner in which the invention may be applied to video signals mayvary with the types of signals being processed. For example, theprocessing of component video signals (Y, Pb, Pr) may be carried out bycalculating the best gain values for the luma component (Y), and thensimply applying the same gain values to the chroma components (Pb, Pr).For S-video and composite signals, the most precise AGC method wouldseparately process gain values for the luma and chroma peaks, in effectperforming the AGC control in parallel for these signals However, it hasbeen found that an effective approach for these signals is to calculatethe best gain values for the luma signal according to the preferredembodiment of the invention as described below, and then calculate theAGC gain values for the chroma signals based only on color burstamplitude. It is contemplated that these, and other, alternativeimplementations of the preferred embodiment of the invention will beapparent to those skilled in the art having reference to thisspecification, and that these alternative implementations are within thescope of this invention as claimed.

FIG. 4 is a flow diagram illustrating the operation of analog front-end15 in updating the current gain control value G_(n-1) to its nextiterative value G_(n), according to the preferred embodiment of theinvention. Coarse AGC stage 16 may also similarly adjust its output(more coarsely, of course) in the conventional manner; however, thispreferred embodiment of the invention is primarily based on the fineadjustment and control of AGC gain, and as such will generally assumethat the coarse gain value N is essentially constant. In this exemplaryimplementation, analog front-end function 15 continually updates itsgain control value G_(n) on a frame-by-frame basis, based on the contentof the video input signal for the most recently received and processedframe n. The applied gain control value G₀ for the first frame may be anarbitrary value, perhaps in the mid-range of the available gains; thegain control value G_(n) for later frames will be derived by aniterative adjustment from the previous frame. The flow diagram of FIG.4, in combination with FIG. 3, illustrates the operation of this gainvalue updating for an exemplary iteration of this operation.

It is contemplated that the updating of AGC gain according to thisembodiment of the invention may be effected within the vertical blankinginterval after the receipt and processing of each frame, so that the newgain value may be applied to the next incoming video frame.Alternatively, it is contemplated that the updating of the gain valuemay be performed on either a more frequent or a less frequent intervalthan after each frame. For example, if sufficient processing capacity isavailable, the gain updating may be performed on a field-by-field basis,or even after every one or several scan lines, to provide even higherfidelity AGC; conversely, if lesser fidelity can be tolerated, cost canbe saved by updating the AGC gain after every two (or more) frames. Itis contemplated that those skilled in the art having reference to thisspecification will be readily able to select the appropriate updatinginterval. However, it is also contemplated that updating at the framerate will be optimum in most modern applications of this invention.

Referring to FIG. 4, a frame n of video input signals is received andmeasured in process 50. Referring back to FIG. 3, this frame n hasalready been amplified by front-end AGC stage 20 using the current gainvalue G_(n-1), and has been clipped as necessary by clip function 42according to the particular clipping levels in place. Various attributesof the amplified and clipped video input signals, over frame n, are thenmeasured from this received frame, by gain update function 46, incombination with the necessary associated circuitry within video decoder10, in processes 52. In addition, as shown in FIG. 3, one or moreattributes of the video input signal for frame n are measured in process52 d at a point, in this example, after luma and chroma separation byseparation and filtering function 26 (FIG. 2). According to thisembodiment of the invention, the multiple attributes of the input videosignal that are measured in processes 52 include attributes of thesignal that are factors in the brightness of the image and in thequality of the video signal itself. Some of these factors areindependent of the image to be displayed, and are contained within thehorizontal blanking interval between successive scan lines of videosignal. Some of the factors indirectly correlate to the output signallevel, while others directly correlate to the output.

FIG. 5 illustrates an exemplary waveform of the horizontal blankinginterval in an NTSC television signal. As known in the art, thehorizontal blanking interval is a portion of the video signal waveformfollowing the image signal for one scan line, and contains signalportions that are used to synchronize the receiving system from a timingand amplitude standpoint. In this NTSC horizontal blanking intervalshown in FIG. 5, horizontal sync pulse HSYNC is a low level portion ofthe horizontal blanking waveform following the image data for a scanline, and has a desired level of −40 IRE units relative to the so-called“blanking level” of 0 IRE units in the NTSC standard. Horizontal syncpulse HSYNC is preceded by a brief “front porch” pulse FP which isnominally at the blanking level of 0 IRE units. Color burst BURST iscontained within the so-called “back porch” BP, and consists of anamplitude modulated continuous sine wave at the chroma signal frequencyof 3.579545 MHz, and nominally has an amplitude of ±20 IRE units. Backporch BP refers to the portion of the horizontal blanking waveform fromthe rising edge of the horizontal sync pulse HSYNC to the beginning ofactive video, and includes the color burst BURST.

In process 52 a, the attribute measured is the “sync height” of thevideo signal. In this example, the sync height attribute refers to theamplitude of the horizontal sync tip level relative to the so-called“back-porch” level in the horizontal blanking interval. Accordingly, the“sync height” measured in process 52 a is the difference between thelevel of the horizontal sync pulse HSYNC and the back porch level BP.Process 52 b measures another image-independent attribute in thehorizontal, blanking interval, namely the amplitude of color burstBURST. Preferably, the attributes of sync height measured by process 52a and of color burst amplitude measured by process 52 b are measured foreach horizontal blanking interval and averaged over the frame, toaccount for variations within the frame.

Process 52 c measures an attribute of the video signal that depends uponthe image to be displayed, namely the peak amplitude of the compositesignal. According to this embodiment of the invention, this peakamplitude is the highest composite signal amplitude for frame n afterclipping by clip function 42. This peak amplitude is preferably measuredrelative to the level of back porch BP in the horizontal blankinginterval (i.e., 0 IRE units), and is preferably the peak levelencountered anywhere in the frame. This peak composite amplitudemeasured in process 52 c is also measured prior to separation of theluma and chroma signals.

The attributes of sync height, burst amplitude, and composite peakamplitude, each indirectly correlate to the eventual output from videodecoder 10. Process 52 d measures an attribute of the video signalfollowing luma and chroma separation, namely the peak amplitude of theluma signal (i.e., without the modulated chroma, or color differencesignals), within the current frame n. This peak luma amplitude directlycorrelates to the output video signal from video decoder 10, as the lumais the overall brightness of a picture element, without regard to itscolor or hue. As mentioned above, it is contemplated that the updateprocess of FIG. 4 can be performed in real time, before the verticalblanking interval, which permits the measurement of process 52 d afterseparation.

Those skilled in the art having reference to this specification willappreciate that other signal attributes may additionally be measured, ormeasured instead of those described above, in connection with the AGCfunction according to this invention. It is believed, however, that themeasurements of processes 52 a through 52 d will provide a relativelyaccurate indication of the amplitudes of the video signal for a givenframe, for purposes of performing automatic gain control. In addition,measurement processes 52 (and also the corresponding comparisonprocesses 54 to be described below) are shown in FIG. 4 as beingperformed in parallel with one another. This depiction is intended toexpress that the order in which these measurements and comparisons aremade is not important; indeed, these measurements may be made inparallel with one another as shown, sequentially with one another in anyorder, or in some combination. It is contemplated that those skilled inthe art having reference to this specification will be capable of makingthe appropriate measurements of the video signal for the AGC function inan optimal manner, for example as valid information becomes available,for each given implementation of this invention.

In process 54 a, gain update function 46 compares the sync heightmeasured in process 52 a against a desired level. According to thisembodiment of the invention, the result of the comparison of process 54a is a ratio of a nominal, or target, sync height (e.g., 40 IRE units)to the measured (averaged) sync height, as such a ratio is useful in theupdating of the AGC gain level as will be described below. Similarly, inprocess 54 b, gain update function 46 compares the burst amplitudemeasured in process 52 a against a target amplitude, preferablyproducing a ratio of the target amplitude to the measured (averaged)burst amplitude. However, in the event that video decoder 10 cannot lockto a valid color burst, comparison process 54 b preferably returns anull value so that the burst amplitude attribute will not be used to setthe AGC gain value. Process 54 b may not be performed for SECAM inputvideo signals, but is preferably performed for NTSC and PAL input videosignals.

Gain update function 46 compares the peak composite signal amplitudemeasured in process 52 c against a target level to produce acorresponding ratio, in process 54 c. Preferably, process 54 c is alsogated by video decoder 10 locking to a valid color burst; if it is not,comparison process 54 c also returns a null value so that the peakcomposite signal amplitude attribute will not be used to set the AGCgain value.

In process 54 d, gain update function 46 compares the peak lumaamplitude for frame n as measured in process 52 d against its targetamplitude, producing a ratio of the two. This attribute is valid, forpurposes of AGC gain setting, for both monochrome and color inputsignals.

According to this embodiment of the invention, it is preferred thatfront-end AGC stage 20 not apply a gain that causes any portion of thevideo waveform to be clipped. Accordingly, gain update function 46 inthis preferred embodiment of the invention updates the gain levelG_(n-1) to value Gn by selecting, in process 56, the most conservativeone of the comparison ratios derived in processes 54, which will providethe lowest gain. The selected comparison ratio (i.e., the selectedattribute and its measured and target values) is also stored in memory48, in a manner accessible to back-end AGC function 28 as will bedescribed below.

The comparison ratio selected in process 56 is then applied to a gainupdate equation by gain update function 54, in process 58. According tothe preferred embodiment of the invention, the gain update equation is arecursive equation, by way of which the current gain value G_(n-1) ismodified by an error value, typically the error value multiplied by afilter (or convergence) factor, to derive the next gain value G_(n). Thefilter factor is also referred to as a time constant of the recursiveupdating equation. The gain update equation may be considered as a “finecontrol” on the AGC gain. Adjustment of the gain of coarse AGC gainstage 16 may be accomplished by gain update function 46, for example byincrementing the coarse gain if the fine gain value (G_(n)) overflows,and decrementing the coarse gain is incremented if the fine gain valueunderflows. As mentioned above, the coarse AGC gain applied by stage 16is an analog AGC gain, controlled in the conventional manner, while thefine control gain applied by front-end AGC function 20 is applied in thedigital domain.

Many variations on AGC gain equations are known in the art. In general,according to this preferred embodiment of the invention, an updated gainvalue gain(n), based on the previous gain value gain(n−1), can beexpressed as:gain(n)=(1−β)gain(n−1)+β(des _(—) gain)  (3)where the value des_gain is a desired gain that is determined from ameasurement of the signal. For the case of feedback control, in whichthe signal is measured after the application of gain gain(n−1) that isbeing adjusted, value des_gain amounts to a desired multiplier of theapplied gain. For example, if the measured signal is one-half thedesired amplitude, then the value des_gain will correspond to doublingof the applied gain. Conversely, in the feed-forward context (as will bedescribed below), where the signal is measured prior to the applicationof gain gain(n−1) to be adjusted, the value des_gain amounts to anabsolute measure; for example, if the measured signal is one-half thedesired amplitude, the value des_gain will correspond to an absolutegain of two.

In the case of front-end AGC gain stage 20, gain value gain₂₀ isadjusted by feedback control, based on measurement of the signal afterfront-end AGC gain stage 20 (and clip function 42). As noted above,adjustment of gain value gain₂₀ is effected by determining an adjusteddigital AGC gain control value G_(n) which, according to equation (2),will produce an adjusted gain gain₂₀. According to this preferredembodiment of the invention, the adjusted digital control value G_(n) isa linear function of the amplitude reference on which the gain is basedis: $\begin{matrix}{G_{n} = {{- \frac{a_{1}}{b_{1}}} + {\left( {1 - \beta} \right)\left( {\frac{a_{1}}{b_{1}} + G_{n - 1}} \right)} + {{\beta\left( {\frac{a_{1}}{b_{1}} + G_{n - 1}} \right)}\left( \frac{\Delta\quad N_{t}}{\Delta\quad N_{m}} \right)}}} & (4)\end{matrix}$where β is a filter coefficient value serving as a filter factor(ranging between 0 and 1), ΔN_(t) is the target, or desired, value ofthe selected amplitude reference (i.e., of the signal attribute selectedin process 56) and ΔN_(m) is the measured value of the selectedamplitude reference (i.e., the value measured in process 52 for theattribute selected in process 56). As mentioned above, the value a₁ isthe y-intercept of the gain control equation (2), and the value b_(i) isthe slope of the gain control equation (2). For ease of computation, they-intercept value a₁ of this gain control equation (2) may be set tozero, in which case gain update equation (3) simplifies to:$\begin{matrix}{G_{n} = {{\left( {1 - \beta} \right)\left( G_{n - 1} \right)} + {\beta\quad{G_{n - 1}\left( \frac{\Delta\quad N_{t}}{\Delta\quad N_{m}} \right)}}}} & (5)\end{matrix}$or more simply yet: $\begin{matrix}{G_{n} = {G_{n - 1} + {\beta\quad{G_{n - 1}\left( {\frac{\Delta\quad N_{t}}{\Delta\quad N_{m}} - 1} \right)}}}} & (6)\end{matrix}$As especially evident from gain update equation (5), a larger value offilter coefficient value β (i.e., a value closer to 1) permits thetarget-to-attribute ratio to more strongly affect the updated digitalcontrol value G_(n), while a smaller value of filter coefficient value β(i.e., a value closer to 0) permits the target-to-attibute ratio to moreweakly affect the updated digital control value G_(n).

The particular gain update equation used in process 58 preferablyfollows a linear update equation, for example as generally described inequations (4) through (6) above. Other relationships, including higherorder gain update equations, may alternatively be used in process 58 ifdesired. The frame index value n is incremented, in preparation for thenext frame of incoming video signals, and the process is repeated forthis next frame beginning again with process 50.

As discussed above relative to FIG. 2, back-end AGC function 28amplifies the separated signals to apply another gain value, in anautomatic gain control (AGC) manner. According to this preferredembodiment of the invention, back-end AGC function 28 applies its gaingain₂₈ according to a linear gain control equation:gain ₂₈ =a ₂ +b ₂ S _(n-1)  (7)in which a₂ and b₂ are the y-intercept and slope, respectively, and inwhich S_(n-1) is the current digital control value. Also according tothis embodiment of the invention, the back-end AGC gain is updatedaccording to a selected one of multiple signal attributes, so that thecorrection applied by back-end AGC function 28 also avoids miscorrectionof the image amplitude, particularly to avoid saturation. The gainapplied by back-end AGC function 28 thus further ensures that the outputof video decoder 10 is not improperly compensated.

As shown in FIG. 6, back-end AGC function 28 applies a gain valueS_(n-1) to the luma signal Y SIGNAL and the color difference signals CSIGNAL after separation by separation and filtering function 26 (FIG.2), using gain stages 62Y, 62C, respectively. In one exemplaryimplementation of this invention, this back-end AGC gain value S_(n-1)may vary in a range from 1.00 to 2.00. Gain stages 62Y, 62C thus applyAGC gain amplification to the separated luma and color differencesignals, to further correct and compensate for artifacts in the videodecoding process and further improve the fidelity of the displayedimage.

Back-end AGC function 28 also includes gain update function 66, whichreceives the results of the measurements carried out in processes 52 inconnection with front-end AGC function 20, described above. Thesemeasurements include the peak luma amplitude in signal Y SIGNAL, takenprior to gain stage 62Y as shown in FIG. 6. Gain update function 66calculates an updated gain control value S_(n) based upon thesemeasurements, following the general approach described above relative toequation (3). As will become apparent from the following description,the updating of the control value S_(n) is performed in a feed-forwardmanner. The process for updating back-end AGC gain control value S_(n)according to the preferred embodiment of the invention will now bedescribed in detail in connection with FIG. 7.

As in the updating of the front-end AGC gain control value G_(n)described above, multiple attributes of the video signal are also usedto update the back-end AGC gain control value S_(n). In process 72 a,gain update function 66 retrieves the comparison ratio for the attributeselected in process 56 from store 48 (FIG. 3), including both anidentification of which attribute the comparison ratio is associatedwith, and also the value of the selected comparison ratio itself. Inprocess 74 b, gain update function 66 retrieves the color burstamplitude comparison ratio derived in process 54 b for frame n. Inprocess 74 c, gain update function 66 retrieves the comparison ratio ofthe luma peak amplitude for frame n as derived in process 54 d, and inprocess 74 d, gain update function 66 retrieves the comparison ratio forthe sync height amplitude as derived in process 54 a for frame n.According to the preferred embodiment of the invention, the comparisonratio for the measured composite peak amplitude (prior to signalseparation) is not used for updating the back-end AGC gain.

The comparison ratio used in the front-end AGC gain update calculationfor frame n is interrogated in decision 73. If the front-end AGC gaincalculation used the luma peak to update its gain (decision 73 is YES),the back-end AGC gain update calculation sets the updated back-end AGCgain control value S_(n) to a default value of unity, in process 76. Inother words, if the front-end AGC gain compensated for the separatedluma signal already, the back-end AGC gain will not further compensatethe output signal. If the luma peak was not the attribute used in thefront-end AGC gain update (decision 73 is NO), control passes toattribute selection process 74.

In process 74, gain update function 66 selects the signal attribute thathas the smallest comparison ratio (target to measured). As before, sothat back-end AGC function 28 does not saturate the signal, the back-endAGC gain value is adjusted according to the attribute requiring theleast amount of gain. Selection process 74 excludes, from its selection,the attribute that was used in the front-end AGC gain update process, asdetermined from process 72 a. In process 78, the current back-end AGCgain value S_(n-1) is modified according to a gain update equation, aswill now be described in detail.

In its most general sense, the back-end AGC gain update equationinvolves parameters in addition to those involved in the front-end AGCgain equation. This is because the back-end AGC gain update must accountfor updates to the coarse AGC gain stage 16 (if any) and updates to thefront-end AGC gain stage 20. In other words, the next back-end AGC gainvalue gain₂₈(n) will be applied to a frame of video signals to whichupdated gain values gain₁₄(n) and gain₂₀(n) will also have been applied.Accordingly, the back-end AGC gain update equation will depend upon thecurrent and updated front-end AGC gain control values G_(n-1), G_(n),respectively, and also current and updated values of the coarse analoggain control values N_(CG-1), N_(CG), respectively. An generalizedexpression of the back-end AGC gain update equation according to thepreferred embodiment of the invention is: $\begin{matrix}{S_{n} = {{- \frac{a_{2}}{b_{2}}} + {\left( {1 - \beta} \right)\left( {\frac{a_{2}}{b_{2}} + S_{n - 1}} \right)} + {{\beta\left( \frac{1}{b_{2}} \right)}\left( \frac{\Delta\quad N_{t}}{\Delta\quad N_{m}} \right)\left( \frac{\frac{a_{3}}{b_{3}} + N_{{CG} - 1}}{\frac{a_{3}}{b_{3}} + N_{CG}} \right)\left( \frac{\frac{a_{1}}{b_{1}} + G_{n - 1}}{\frac{a_{1}}{b_{1}} + G_{n}} \right)}}} & (8)\end{matrix}$The values ΔN_(t) and ΔN_(m) are the target and measured values,respectively, of the amplitude reference (i.e., signal attribute)selected in process 74. As before, the value β is a filter coefficientthat controls the rate at which the back-end gain control value changesfrom iteration to iteration. In a more typical case, the y-interceptvalues a₁ and a₂ are zero, and equation (4) simplifies to:$\begin{matrix}{S_{n} = {S_{n - 1} + {\beta\left\lbrack {{\left( \frac{1}{b_{2}} \right)\left( \frac{\Delta\quad N_{t}}{\Delta\quad N_{m}} \right)\left( \frac{G_{n}}{G_{n - 1}} \right)\left( \frac{\frac{a_{3}}{b_{3}} + N_{{CG} - 1}}{\frac{a_{3}}{b_{3}} + N_{CG}} \right)} - S_{n - 1}} \right\rbrack}}} & (9)\end{matrix}$Back-end gain update function 66 thus performs, in process 78, therequired calculations to generate updated back-end AGC gain value S_(n),using the appropriate one of equations (8) or (9), or a variationthereof depending upon the particular gain equations to be used. Thisgain value S_(n) is then forwarded to gain stages 62Y, 62C for use inconnection with the next frame to be processed, which will be referredto by the frame index n that is incremented in process 80. The back-endgain update process is then repeated, from processes 72, upon receiptand measurement of that next frame.

According to the preferred embodiment of the invention, additionalperformance improvement may be made available by an optional feature inwhich the rate of change of the front-end and back-end AGC gain controlvalues, and thus the AGC gains themselves, are controlled to moreclosely correspond to image-dependent factors in the video signal. Inthe operation of video decoder 10 described above, the image-dependentsignal attributes of the composite peak amplitude and the luma peakamplitude control the AGC gain only when both the sync height and colorburst amplitudes are below their target levels. In this case, peakvalues in a bright image scene will drive the AGC gain down, and thesync height or burst amplitude attributes will drive the AGC gain backup during dark image scenes. However, if another bright scene appearsafter the AGC gain has been driven high by the non-image-dependentattributes, the output signal will saturate until the image-dependentattributes again drive the AGC gain down. This effect causes the displayof bright image scenes after dark scenes to be unduly brightened by theAGC gain values, resulting in poor fidelity output.

According to the preferred embodiment of the invention, therefore, thefilter coefficient values β for the adjustment of both the front-end andback-end AGC gain update equations are controlled to delay and slow AGCgain increases, following a decrease in the AGC gain caused by imagedependent attributes. The filter coefficient values β for the front-endand back-end AGC gain update equations may be the same, or may differ,depending upon the particular implementation. In any case, the generaloperation of gain update functions 46, 66 to control the rate and delayof gain increases and decreases is similar to one another, and will nowbe described generally with reference to FIG. 8. This process preferablyoperates in conjunction with the gain update processes 58, 78 describedabove, and may be performed by circuitry implemented within AGCfunctions 20, 28, or alternatively elsewhere within video decoder 10,such as digital processor 35. Specifically, it is contemplated that theparticular parameters of the process of FIG. 8, such the various filtercoefficient values β and frame delay values, are programmable by way ofhost interface 36 to digital processor 35, as shown in FIG. 2.

The process of FIG. 8 begins with the receipt of a frame n of videosignal data. Decision 81 determines whether the AGC gain is to beincreased or decreased based on the selected signal attribute and itscomparison ratio. If the gain is to be decreased (decision 81 is NO), aframe counter is reset to zero in process 82, and the identity of thesignal attribute causing this decrease is stored in memory by process84. According to the preferred embodiment of this invention, all AGCgain decreases occur immediately, and at relatively fast rates.Accordingly, a high filter coefficient value β is selected in process86, and is used in the corresponding AGC gain update process 58, 78. Anexample of a high filter coefficient value β for front-end AGC gainupdate process 58 is ⅛, and is unity for back-end AGC gain updateprocess 78. Control then returns to decision 81 to await the next frameof the incoming video signal.

If the AGC gain is to be increased (decision 81 is YES), process 88 isthen performed to retrieve, from memory, the identity of the signalattribute that caused the most recent AGC gain decrease. If theretrieved signal attribute causing the last decrease is an imageindependent attribute (e.g., sync height or color burst amplitude),decision 81 returns a NO result, indicating that the gain increase canoccur immediately and rapidly. Process 92 is then executed to select ahigh filter coefficient value β, for example similar to the filtercoefficient values for AGC gain decreases, and the corresponding AGCgain update process 58, 78 is then performed. Control then passes todecision 81 to again await the next frame of the incoming video signal.

If, on the other hand, the attribute causing the most recent AGC gaindecrease is an image dependent attribute (e.g., composite signal peakamplitude, or luma signal peak amplitude), decision 89 returns a YESresult. The frame counter is then incremented, in process 92, and itscontents compared against a delay limit value in decision 93. The delaylimit value is contemplated to be a programmable value indicating thenumber of frames for which indicated gain increases are to be delayed,and may vary, for example, from zero to 255, with a typical value ofabout 30 frames. If the number of gain increase frames counted by theframe counter has not yet reached the delay limit value (decision 93 isNO), then no AGC gain update is performed, and control returns todecision 81 to await the next frame of incoming video signals.

If the delay limit value is reached by the frame counter (decision 93 isYES), process 94 is executed to select a low filter coefficient value βfor use in the appropriate AGC gain update process 58, 78. Examples oflow filter coefficient values β range from unity to {fraction (1/128)},with a typical value being on the order of {fraction (1/64)}. Obviously,the low high filter coefficient value β will be lower than the highfilter coefficient value β used for gain decreases, and gain increasesfollowing non-image-dependent gain decreases. Following selectionprocess 94, the corresponding AGC gain update process 58, 78 isperformed, and control returns to decision 81 to await the next frame ofincoming video signals.

According to this preferred embodiment of the invention, therefore, therate at which the AGC gain is increased depends upon the reason forwhich the AGC gain was most recently decreased. If the decrease in AGCgain was due to image-dependent signal attributes, successive AGC gainincreases are delayed and slowed. This improves the fidelity of thevideo image sequences, by reducing the likelihood of image saturationdue to bright images following a short sequence of relatively darkimages in the signal. The resulting overall video result is thereforeimproved over that provided by conventional video decoders.

According to this preferred embodiment of the invention, therefore, theuse of multiple attributes of the incoming video signal providesimproved fidelity in the output video signal that is forwarded to theeventual video display. Over-correction, for example in the form of overamplifying the signal resulting in saturated “white” peaks in thedisplayed image, is avoided to a larger extent. In addition, theapplication of both front-end (pre-separation) and back-end(post-separation) AGC functions, both of which have their gains updatedaccording to a selected one of the multiple signal attributes, providesimproved overall gain control and thus improved fidelity in thedisplayed images. Also according to the preferred embodiment of theinvention, both image-dependent and image-independent attributes aremeasured and may be used for gain control; these attributes also includethose attributes that directly correlate to the output video signal, andthose that only indirectly correlate to the output. And, as mentionedabove, the rate and delay of gain increases can be controlled accordingto the cause of the most recent gain decrease, further preventing thelikelihood of image saturation at the display. This invention thusprovides a robust and high-fidelity approach to automatic gain controlin the video decoder context.

While the present invention has been described according to itspreferred embodiments, it is of course contemplated that modificationsof, and alternatives to, these embodiments, such modifications andalternatives obtaining the advantages and benefits of this invention,will be apparent to those of ordinary skill in the art having referenceto this specification and its drawings. It is contemplated that suchmodifications and alternatives are within the scope of this invention assubsequently claimed herein.

1. A video decoder, comprising: input interface circuitry for receivingan input video signal; separation circuitry, for separating the receivedinput video signal into signal components; output format circuitry forpresenting an output signal to a video display; and a first automaticgain control circuit, comprising: a gain stage for amplifying a signalcorresponding to the input video signal by a first gain value; and gainupdate circuitry, for modifying the gain value responsive to a selectedone of a plurality of signal attributes of the input video signal. 2.The video decoder of claim 1, wherein the plurality of signal attributescomprises: at least one signal attribute that is dependent upon thecontent of an image to be displayed responsive to the input videosignal; and at least one signal attribute that is not dependent upon thecontent of the image to be displayed.
 3. The video decoder of claim 2,wherein the plurality of signal attributes comprises: at least onesignal attribute selected from the group consisting of sync height,color burst amplitude, composite peak amplitude, and luma peakamplitude.
 4. The video decoder of claim 1, wherein the input videosignal includes a vertical blanking interval between frames of the inputvideo signal; wherein the gain update circuitry modifies the first gainvalue responsive to measurements of the plurality of signal attributesover a received frame of the input video signal.
 5. The video decoder ofclaim 1, wherein the first automatic gain control circuit has an outputcoupled to an input of the separation circuitry.
 6. The video decoder ofclaim 5, further comprising: a second automatic gain control circuit,having at least one input coupled to a corresponding output of theseparation circuit, and comprising: at least one gain stage foramplifying a signal corresponding to the output of the separationcircuitry by a second gain value; and gain update circuitry, formodifying the second gain value responsive to a selected one of aplurality of signal attributes of the input video signal.
 7. The videodecoder of claim 6, wherein the plurality of signal attributes to whichthe gain update circuitry of the second automatic gain control circuitis responsive differs from the plurality of signal attributes to whichthe gain update circuitry of the first automatic gain control circuit isresponsive.
 8. The video decoder of claim 7, wherein the plurality ofsignal attributes, to which the gain update circuitry of the secondautomatic gain control circuit is responsive, comprises the selected oneof the plurality of signal attributes to which the gain update circuitryof the first automatic gain control circuit modified the first gainvalue for a frame of the input video signal.
 9. The video decoder ofclaim 8, wherein the selected one of the plurality of signal attributes,for each of the first and second automatic gain control circuits,corresponds to one of the plurality of signal attributes resulting inthe lowest gain.
 10. The video decoder of claim 9, wherein the secondautomatic gain control circuit excludes the selected one of theplurality of signal attributes for the first automatic gain controlcircuit from being its selected one of the plurality of signalattributes.
 11. The video decoder of claim 9, wherein one of theplurality of signal attributes to which the gain update circuitry of thefirst automatic gain control circuit is responsive is a luma peakamplitude; and wherein the second automatic gain control circuitmodifies the second gain value to a default value responsive to theselected one of the plurality of signal attributes for the firstautomatic gain control circuit being the luma peak amplitude.
 12. Thevideo decoder of claim 1, wherein the gain update circuitry modifies thefirst gain value based on measurements of the plurality of signalattributes for each frame of the input video signal, by eitherincreasing or decreasing the first gain value; wherein the plurality ofsignal attributes comprises: at least a first signal attribute that isdependent upon the content of an image to be displayed responsive to theinput video signal; and at least a second signal attribute that is notdependent upon the content of the image to be displayed; wherein thegain update circuitry modifies the first gain value responsive to a gainupdate equation including a filter coefficient value, higher values ofthe filter coefficient value corresponding to greater changes in thefirst gain value; wherein the gain update circuitry comprises a memoryfor storing an identity of an attribute causing a most recent decreasein the first gain value; wherein the gain update circuitry increases thefirst gain value using a first filter coefficient value responsive tothe identity of the attribute causing the most recent decrease in thefirst gain value being the second signal attribute; and wherein the gainupdate circuitry increases the first gain value using a second filtercoefficient value, lower in value than the first filter coefficientvalue, responsive to the identity of the attribute causing the mostrecent decrease in the first gain value being the first signalattribute.
 13. The video decoder of claim 12, wherein the gain updatecircuitry delays increasing the first gain value for a selected numberof frames responsive to the identity of the attribute causing the mostrecent decrease in the first gain value being the first signalattribute.
 14. A method of controlling automatic gain control in a videodecoder, comprising the steps of: measuring a first plurality of signalattributes of an input video signal; comparing each of the signalattribute measurements to a corresponding target value for the signalattribute; selecting one of the first plurality of signal attributesresponsive to the comparing step; modifying a first gain value accordingto the selected signal attribute measurement.
 15. The method of claim14, wherein the first plurality of signal attributes comprises: at leastone signal attribute that is dependent upon the content of an image tobe displayed responsive to the input video signal; and at least onesignal attribute that is not dependent upon the content of the image tobe displayed.
 16. The method of claim 15, wherein the first plurality ofsignal attributes comprises: at least one signal attribute selected fromthe group consisting of sync height, color burst amplitude, compositepeak amplitude, and luma peak amplitude.
 17. The method of claim 15,wherein the input video signal is arranged in frames; wherein themeasuring step measures the first plurality of signal attributes foreach frame; and wherein the comparing, selecting, and modifying stepsare performed for each frame, using the signal attribute measurementsfor that frame.
 18. The method of claim 17, wherein the modifying stepmodifies the first gain value according to a gain update equationincluding a filter coefficient value, higher values of the filtercoefficient value corresponding to greater changes in the first gainvalue and further comprising: after the selecting step, determiningwhether the modifying step is to increase or decrease the first gainvalue for a frame; responsive to the modifying step decreasing the firstgain value for a frame, storing the identity of the selected signalattribute for that frame; responsive to determining, for a subsequentframe, that the modifying step is to increase the gain value: retrievingthe stored identity of the selected signal attribute; responsive to theretrieved selected signal attribute corresponding to a signal attributethat is dependent upon the content of an image to be displayedresponsive to the input video signal, modifying the first gain valueusing a first filter coefficient value; and responsive to the retrievedselected signal attribute corresponding to a signal attribute that isnot dependent upon the content of an image to be displayed responsive tothe input video signal, modifying the first gain value using a secondfilter coefficient value, the second filter coefficient value beinghigher than the first filter coefficient value.
 19. The method of claim18, further comprising: responsive to determining, for a subsequentframe, that the modifying step is to increase the first gain value andresponsive to the retrieved selected signal attribute corresponding to asignal attribute that is dependent upon the content of an image to bedisplayed responsive to the input video signal, delaying the step ofmodifying the first gain value for a selected number of frames.
 20. Themethod of claim 14, further comprising: applying the first gain value toa signal corresponding to the input video signal.
 21. The method ofclaim 20, further comprising: after the applying step, separating theinput video signal into component signals.
 22. The method of claim 21,further comprising: after the separating step, applying a second gainvalue to the component signals; and modifying the second gain valueaccording to the selected signal attribute measurement.
 23. The methodof claim 22, further comprising: determining the first selected signalattribute from the first plurality of signal attributes; selecting oneof a second plurality of signal attributes, the second plurality ofsignal attributes including the first selected signal attribute; whereinthe selecting step excludes the first selected signal attribute frombeing selected.
 24. The method of claim 22, wherein the first pluralityof signal attributes comprises a luma signal peak amplitude; and furthercomprising: responsive to the first selected signal attribute being theluma signal peak amplitude, modifying the second gain value to a defaultvalue.